1. Field of the Invention
This invention relates to an asynchronous high-speed data interface for processing serial data frames and, more particularly, to such an interface for processing frames originating from a serial channel and destined for a control unit having a parallel interface.
2. Description of the Related Art
The copending application of D. F. Casper, J. R. Flanagan, G. H. Miracle, R. A. Neuner and P. L. Potvin, Ser. No. 07/392,754, entitled "Apparatus for Interconnecting a Control Unit Having a Parallel Bus with a Channel Having a Serial Link", and the copending application of M. E. Carey, G. H. Miracle, J. T. Moyer, and R. A. Neuner, Ser. No. 07/392,629 entitled "Channel and Extender Unit Operable with Byte Mode or Non-Byte Mode Control Units", both owned by the assignee of this application and filed on even date herewith, describe a fiber-optic-extended parallel channel emulator. As described in those applications, the total extended channel consists of a cooperating serial channel, associated with a host processor, which is connected through a serial fiber-optic link to an extender unit coupled to one or more parallel control units. The control units in turn handle devices, especially direct-access storage devices (DASD) such as magnetic disk drives. The serial protocols for communicating over the fiber-optic link and the format of the serial frames transmitted over the link are also described in the above-identified copending application of D. F. Casper et al. The use of the serial fiber-optic link between the serial channel and the extender unit allows control units to be placed up to 3 km from the channel (1 km in the case of certain time-critical control units), as contrasted with a distance of about 400 feet for conventional parallel channels and control units.
In the system described in the copending applications referred to above, the channel and the extender unit coupled by the fiber-optic link are timed by clocks which, although having the same nominal frequency, run asynchronously with each other. In the extender unit, frames arriving from the channel are received under the control of a clock that is synchronous with the channel transmitter clock and are later processed under the control of a clock that is used generally to time the extender unit logic and is asynchronous with the channel transmitter clock. Such asynchronism between the receiving of the serial frame and its later processing usually requires that the entire frame be received in an input buffer and checked for validity before the frame is further processed. This requirement significantly limits the data transmission rate and may result in an input buffer overrun if two consecutive frames are spaced two closely together. Further, the necessity of receiving the entire frame in the input buffer before proceeding with later processing increases the effective latency period of the device connected to the control unit.